Given two terms tm1 and tm2, a call term_unify vars tm1 tm2 attempts to
find instantiations of the variables vars in the two terms to make them
alpha-equivalent. At present, no type instantiation is done. The unification is
also purely first-order. In these respects it is less general than
term_match, and this may be improved in the future.

FAILURE CONDITIONS

Fails if the two terms are not first-order unifiable by instantiating the given
variables without type instantiation.